Typically, printed circuit boards are protected from moisture, electric leakage and dust with a coating of a moisture proof insulative film, known as a conformal coating, such as an acrylic, polyurethane, silicone or epoxy synthetic resin. Since the coatings are often dissolved in volatile solvents, the coated circuit boards are usually transported through a flash chamber to flash off the volatile solvents prior to curing. Then, the coated circuit boards are transported through a curing chamber and subjected to ultra-violet ("UV") light to initiate the polymerization process for curing the conformal material. The UV lights, while necessary for curing the conformal coating material, have significant disadvantages, primarily relating to the considerable amount of heat which they emit.
In a high volume production process, such as typically used in manufacturing circuit boards, two or more UV lights are often employed to cure the conformal coating material. Because of the considerable heat that these lights generate, there are problems relating to the rate of temperature rise and the maximum temperature of the circuit boards within the curing chamber.
Concerning the rate of temperature rise, there is a chance that the electronic components mounted on the circuit board can be damaged. In particular, when the electronic components are surface mounted to the circuit boards, significant stresses can be generated in the solder joints mounting the components due to the rapid rise in temperature. These stresses often lead to fracture of the joints and sometimes cause components to pop off the board. Moreover, in some circumstances, these stresses can damage, e.g. fracture, the circuit board itself. Therefore, the maximum rate of temperature rise is typically specified to be about 2.degree. centigrade per second. However, the specific rate can vary depending upon the particular application and materials.
The other concern is to insure that the electric or electronic devices mounted to the circuit boards do not exceed a specified maximum temperature during manufacture. The temperature which the electronic devices attain relates to the temperature within the curing chamber and the amount of time that the circuit board stays within the curing chamber. If the temperature that the electronic devices reach is too high, typically above about 85.degree. centigrade, the electronic components can be damaged. Here again, the temperature limitations are dependent upon the specific materials and components forming the circuit board assemblies.
Another important factor to be considered in curing conformal coatings is the importance of driving off any volatile solvents before initiating the curing process. If solvents remain in the conformal material during the curing step, they can produce bubbles in the cured coating and degrade the electrical and insulative properties of the completed board. In some cases, the coated boards are simply stored until the solvents flash off, typically several minutes to several hours. To reduce the time required to flash off the volatiles, circuit boards are sometimes subjected to heat, generated by means such as electric heaters. However, this is costly and reduces the profit for each completed circuit board.
Another problem with the prior art systems is that the conformal coatings applied on printed circuit boards to protect them from moisture, electric leakage and dust often do not have a uniform thickness. For example, an uneven coating can result when the coating is applied by a spray gun of the type disclosed in U.S. Pat. No. 5,141,165, which is hereby incorporated by reference in its entirety. Therefore, even when the volatile solvents, typically dissolved in the conformal coatings, are completely flashed off before curing, the final cured coating can still have poor performance because some areas are too thin and therefore unable to provide desired electrical and insulative properties. While simply increasing the thickness of the coating being applied can solve this problem, the cost of the additional conformal material and the impact on the environment from the release of the extra solvents makes this only a partial solution to the problem.